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Neornithes: Running Out of Time

A
surprising number of recent papers in avian evolution still treat the Cretaceous
evolution of birds as if it were a stately, linear progress from waddling
dinosaurs to the near-miracles of neornithine flight engineering. Of
course paleontologists know better. Evolution is never "toward"
anything in a teleological sense. However, it is admittedly hard to keep
this in mind at all times. Our every-day experience is of eagles and
swifts and other birds so highly evolved for flight that it seems absurd to
think they might ever have been specialized for anything else.

However, just as an exercise, we started running (or flying, as
the case may be) down the list
of probable Cretaceous neornithines. The Cretaceous was full of flying
birds, and also appears to have been full of various neornithine birds. The
interesting point is that there were rather few flying neornithine birds
from the Cretaceous. This requires a little qualification. The
neornithines evolved from potentially volant avian stock. So it is likely
that the earliest neornithines flew. For that reason we exclude the Lithornithiformes
who seem to have been at or close to the very base of the neornithine radiation.
However, it is worth noting that a remarkable proportion of other birds close to
the base of Neornithes were non-flyers: e.g., Patagopteryx, Hesperornis,
Otogornis (Hope, 2002).

But, after Lithornis and company, we have the
unclassifiable mihirung (Dromornithifirmes) and
the Phorusrhachidae, both of which probably had Cretaceous roots. Both
could probably fly as well as a helium-filled hippopotamus. Then we might
list the tinamous (Tinamiformes), which can become
airborne in
emergencies, but are famous for flying full-tilt into trees and breaking their
necks when they do so. Then all the rest of the ratites,
none of whom can fly any better than we can. But how about the
neognaths? The Cretaceous probably had some early galliforms.
Chickens can fly a bit, usually better than the tinamous; but many turkeys
can't even manage this. Megapodes
and the like are accomplished runners, but also weak flyers. On the anseriform
side, we do better [1]. There are still a number of flightless forms, but most
ducks and geese are passable flyers, although more highly adapted to water than
air. Further, we can be certain that duck-like birds (Vegavis) were
present in the very latest Cretaceous. Clarke
et al. (2005). However, until the very end of the Cretaceous,
the number of fully volant neornithines seems remarkably small.

Contrast this spotty record with the main competitors of
Neornithes: the Enantiornithes.
The argument that enantiornithines were poor flyers is no longer tenable. Chiappe
& Walker (2002). In fact, we can now stand the argument on its
head. If, as seems to be the case, the basic enantiornithine body plan is
volant, what is the evidence that any enantiornithine was flightless? As
far as we are aware (a major reservation!) no such evidence exists. Thus,
we cannot blame the fossil record for all those earth-bound
neornithines. The overwhelming weight of the evidence -- and of the
birds, for that matter -- is that the neornithines initially specialized in various niches
which required little or no flight.

Finally, we might speculate that all this running around was the ultimate cause of neornithine success. As in acting,
good bones and staying thin will only get you so far. To really make use
of the talent, you have to have high energy. The neornithines kept close
enough to flight [2] to retain the good bones and, (with some exceptions -- see
image) they kept thin. What the running lifestyle may have given them was
the more completely endothermic metabolism needed to make good use of the
ability to fly. So, when neornithines returned to the air in the Latest
Cretaceous, they were pre-adapted for a whole new range of high-energy flying
behaviors not open to the Enantiornithes, such as migration, predation on the
wing, or extended care of young. ATW051030

Speaking of running around,
Tsuda et al.
(2007) have recently reviewed and extended some work on the sex chromosomes
of basal neornithines. The bird sex-determination system is the reverse of
the mammalian system. Females are heterozygotic, having both Z and W
chromosomes. Males are ZZ. As Tsuda et al. reconstruct the
history, early neornithines had sex chromosomes (the W and Z chromosomes in
birds) which were very similar, differing not much more than ordinary pairs of
somatic chromosomes in size and gene content. This similarity is retained
to some extent in tinamous and
ratites. In both of these groups, some
Z-specific ("male") genes are deleted on the W chromosome, the gaps being filled
by repetitive, non-coding sequences. Oddly enough, the patern of deletion
seems to begin near the middle of the chromosome and work outward over the
course of bird evolution. In neognathous birds (all living birds except
ratites and tinamous), the Z and W chromosomes are highly differentiated.

This finding tends, if slightly, to confirm our speculation above. That
is, it provides another indication that the primitive condition for neornithines
was similar to that of ratites and tinamous: with only slight sexual dimorphism
and, so, perhaps also a primarilly ground-based, non-flying mode of life.
ATW080115.

Notes:[1] We exclude the screamers. The Anhimidae
are quite good flyers today, but have a number of characteristics so weird that
we question whether their immediate Cretaceous ancestors could fly at all: lack
of apteria, ground-nesting, large number of cervical vertebrae, hallux low, very
large feet.

[2] Ostriches and a few other non-flying
birds with substantial wings seem to spread their wings as each leg passes the
peak of the swing
phase at in the running cycle (pers. obs.). This makes good
mechanical sense, since the air resistance of the wings spread horizontally (a)
reduces the ground reaction force (b) increases the time in the air, (c) may
contribute to balance and maneuverabiliity, and (d) allows the the bird to
remain more stiff-legged which, in turn, improves mechanical advantage (Hutchinson,
2004). At some point, wing reduction may become so pronounced that
this sort of effect is no longer significant. However, until then, the
interplay between limb and wing in fast locomotion may lead back to
flight.

Characters: The
more important anatomical characteristics of this group are discussed in the Overview.
They include: horn beak; teeth absent; fused limb bones. In addition Neornithes
have a fully-separated four-chambered heart and typically exhibit complex social
behaviors.

Characters:$ "Paleognathous" palate, consisting of: large
(pre-?)vomer attached to pterygoid and palatines, excluding both from
parasphenoid joint between , no pterygoid and palatine; moveable joint between braincase
and pterygoid, and large basipterygoid
processes; primitively, splenial present
on dentary; numerous
"flight" characters of skeletal anatomy are generally preserved, if
more or less vestigialized. May be polyphyletic -- various phylogenetic
possibilities discussed above. Most relevant characters are likely
primitive for neornithines. For example, the paleognathous palate is known
from Hesperornis.
Thus, there are no definite synapomorphies of this group. The figure at right is
loosely based on a stylized rhea.